1. Field of the Invention
The present invention relates to a linear vibration motor using resonance frequency. More particularly, in a vertical vibrator which does not use a conventional vibration motor mechanism based upon brushes and a commutator, the linear vibration motor of the invention can minimize the volume of an elastic member except for the route of movement in order to maximize the amplitude of vibration in a fixed volume.
2. Description of the Related Art
Communication instruments generally have a call receiving function as one of their essential functions. Typically, the call receiving function generates sound such as a melody and bell or vibrates corresponding communication instruments.
Since the melody or bell may disturb others when transferred to the outside via a speaker, the vibration function is used generally to avoid this situation. In order to enable the vibration function, a small sized vibration motor is generally actuated transferring a driving force to a housing of a communication instrument thereby to vibrate the same.
Vibration motors currently utilized in mobile phones are classified into thin coin and elongate bar types according to their configurations.
FIG. 1 is a perspective exploded view illustrating a conventional coin type vibration motor in detail, which will be described as follows.
A coin type vibration motor 100 generally includes a stator assembly 110 in the form of a stationary member and a rotor assembly 120 in the form of a rotary member.
The stator assembly 110 has a bracket 111 in the form of a circular plate, a lower board 112 attached on the top of the bracket 111 and an annular magnet 113 concentrically attached on the top of the bracket 111 around the lower board 112 in the same fashion.
The bracket 111 is enclosed by a housing 150 from above, and a central shaft 130 is connected between the bracket 111 and the housing 150.
The rotor assembly 120 is rotatably placed around the shaft 130, and has a bearing 121, a coil assembly 122, a counter weight 123, a commutator 124, an upper board 125 and an insulator 126.
The stator assembly 110 is electrically connected to the rotor assembly 120 via brushes 140 that are fixed at the bottom ends to the lower board 112 and the top ends to the commutator 124.
FIG. 2 is a sectional view of a conventional bar type vibration motor, which will be described as follows.
A bar type vibration motor 200 generally includes a stator assembly 210 and a rotor assembly 220.
The stator assembly 210 has a body 211, a stationary cap 212 fixed to the body 211 and a magnet 213 fixed to the body 211. The body 211 is of a hollow cylindrical member, and fixedly houses the magnet 213 therein.
The rotor assembly 220 has an eccentric weight 223, a stationary member 225, a commutator 224, which is fixedly attached on one side of the stationary member 225 and divided into a plurality of segments, and a plurality of coil assemblies 222 fixed to the stationary member 225.
A lower board (not shown) mounted on the stationary cap 212 has a pair of brushes 240 fixed thereto, in which the brushes 240 are connected to power-supplying lead wires 214 and contact the commutator 224 to apply voltage thereto.
Regardless of their designs, the above vibration motors generate rotational force to turn a rotary unit having an unbalanced mass thereby obtaining mechanical vibration, in which the rotational force is generally produced by supplying voltage to a rotor coil through the commutation by contacts of the brushes and the commutator.
Unfortunately, a brush type vibration motor incorporating such a commutator has following problems: In the rotation of the motor, brushes passing through a gap between segments of a commutator creates mechanical friction and an electric spark so that the brushes and the commutator are abraded to shorten the lifetime of the motor.
As a vertical vibrator in the form of a brushless vibration motor for overcoming some drawbacks of such a brush type vibration motor, a portable vibrator is proposed in Japanese Patent Publication Serial No. 2003-117489. The vertical vibrator is illustrated in FIG. 3, and will be described herein as follows.
A cylindrical frame 410 is provided at axial ends with a pair of brackets for supporting both ends of a stationary shaft, and a cylindrical coil 420 having terminals is fixed to the inner periphery of the cylindrical frame 410. A cylindrical permanent magnet 430 is radially magnetized, and elastic members 440 are fixed coplanar with the permanent magnet 430.
Generally in a vibration motor using resonance frequency, the amplitude of vibration F is expressed as Equation 1 below:F∝M·X·f2  Equation 1,
wherein M is the mass of a vibrator, X is the displacement of the vibrator and f2 is the square of resonance frequency.
That is, the amplitude of vibration increases in proportion to the mass of the vibrator in a fixed volume.
However, the portable vibrator 400 as shown in FIG. 3 also has following problems: The coil 420 is placed on the inside wall of the housing occupying a large space, and the elastic member is of a coil elastic member (i.e., an elastic member in which its plates are accumulated in thickness to occupy a volume if extruded to the maximum extent) so that the volume occupied by the elastic member except for the track drawn by its movement does not provide a space for increasing the volume of the vibrator in a fixed volume. That is, the mass of the vibrator cannot be maximized failing to maximize the amplitude of vibration in a fixed volume.
As a consequence, vibration motors using resonance frequency capable of preventing such problems have been required in the art.